Techniques of impurity diffusion are important as those to be utilized in producing semiconductor devices such as semiconductor lasers or light emission diodes.
FIG. 3 shows a prior art production method of a laser diode utilizing diffusion of p type and n type impurities. In FIG. 3, reference numeral 1 designates a semi-insulative GaAs substrate. A high resistance AlGaAs layer 9 is produced on the semi-insulative GaAs substrate 1. A superlattice active layer 2 comprising GaAs and AlGaAs layers is provided, put between the high resistance AlGaAs layers 9. An n type and a p type diffusion region 5 and 6 are produced by impurity diffusion, respectively. A SiN film 4 is producted on the high resistance AlGaAs layer 9 as a mask for impurity diffusion for producing the n type impurity diffusion region 5. A SiN film 11 is produced on the high resistance AlGaAs layer 9 as a mask for producing the p type diffusion region 6. Reference numeral 7 designates a diffusion front of the n type diffusion region 5. Reference numeral 12 designates a diffusion front of the p type impurity diffusion region 6. Reference numeral 8 designates an active region of the superlattice active layer 2.
The production method of this laser diode will be described.
First of all, on the (100) main surface of a semiconductor wafer having a superlattice layer 2, a SiN film 4 as a diffusion mask is produced to a thickness of 700 to 1000 Angstroms by thermal chemical vapor deposition (CVD) or low pressure CVD. Thereafter, a window of stripe photolithography. For the etching of the SiN film 4, a plasma etching using an etching gas mainly consisting of CF.sub.4 gas is used. Selective diffusion of n type impurities is conducted only into the window region of the semiconductor wafer having such a window region by a closed tube method, thereby producing an n type diffusion region 5. In the diffusion, Si is used as diffusion source of n type impurties and metal As is generally used to produce As ambient for preventing thermal decomposition of GaAs wafer, that is, dispersion of As into the ambient due to the thermal processing. The diffusion is conducted at a temperature of about 850 degrees centigrade. In the superlattice layer 2 at the n type diffusion region 5, disordering of crystals occurs, thereby producing mixture of crystalline GaAs and AlGaAs.
Next, the SiN film 4 which has been used as a diffusion mask is removed and a SiN film 11 as a second diffusion mask is grown on the surface of the semiconductor wafer. A window aperturing is again executed to the SiN film 11 by photolithography in a state where the n type diffusion region 5 is covered by the film. Then, it is required that the diffusion front 7 and the end of the SiN film 11 are spaced-apart from each other by several microns. Because the diffusion front 7 cannot be usually seen from the surface of the semiconductor wafer, an extra process such as etching for making the diffusion front 7 easy to be seen is required. This etching is conducted by using mixed solution of hydrofluoric acid, hydrogen peroxide, and water, and the diffusion front can be recognized when step is produced between the n type diffusion region 5 and the high resistance AlGaAs layer 9 due to the difference in the etching rates. Even after the diffusion front 7 is made easy to be seen from the surface of the semiconductor wafer, the diffusion front 7 is rather difficult to be recognizd and it is quite difficult to conduct mask alignment with positioning the end of the SiN film 11 at a position by several microns apart from the diffusion front 7. After this patterning, diffusion of Zn is conducted by such as a closed tube method to produce a p type diffusion region 6. Metal Zn is used as a diffusion source and metal As is used for preventing thermal decomposition of GaAs wafer. The diffusion temperature is made about 700 degrees centigrade. In the superlattice active layer 2 at the p type diffusion region 6, disordering of crystal also occurs.
After the diffusion is concluded, electrodes (not shown) are produced on the n type and p type diffusion regions, respectively. When a bias is applied to the semiconductor laser with making the p type diffusion region 6 biased in a forward direction, a current flows through the p type diffusion region 6, the active region 8, and the n type diffusion region 5. The active region 8 is surrounded by the AlGaAs layers at above and below, and by disordered AlGaAs layers at left and right, and thus a so-called BH (buried heterostructure) laser capable of realizing a low threshold curent and a fundamental tansverse mode oscillation is obtained.
In the prior art production method of a semiconductor device of such a construction, the positional alignment of the second diffusion mask is quite difficult, and this made it quite difficult to obtian high performance device with a high reproducibility.